Epoxy resin compositions are useful to encapsulate electronic components, and as structural adhesives, and the like. Reinforced epoxy resin composites having high strength to weight ratios have found extensive use in the aircraft and aerospace industries, and in other applications where strength, corrosion resistance and light weight are desirable. For instance, fiber resin matrix materials have replaced aluminum and other metals in primary and secondary structures of modern military and commercial aircraft. Sporting equipment such as tennis rackets and golf clubs have also adopted fiber resin materials successfully.
Epoxy resin compositions and fiber modifications are abundant. Since the advent of fiber resin matrix materials, much effort has been expended in improving their properties and characteristics, including the development of many curing systems.
It is known to add curing agents functioning as accelerators or catalysts such as substituted ureas and boron trifluoride-amine complexes to such epoxy compositions to cure them, and also to significantly decrease the hardening temperature of such resin systems. Substituted ureas made from monoamines and polyisocyanates and used for this purpose are described, for example, in U.S. Pat. Nos. 3,386,955 and 894,113 as well as in U.S. patent application Ser. No. 518,879, filed Aug. 1, 1983, U.S. Pat. No. 4,558,078, issued Dec. 10, 1985, and the use of boron trifluoride-amine complexes is also described and claimed in the latter. While such curing agents do lower gel times, they tend to lower the mechanical properties of high performance epoxy resins, especially those containing tetraglycidyldiaminodiphenylmethane. Although they increase resin toughness, they lower hot/wet properties and glass transition temperatures.
Amine and polyamine curing agents have received wide acceptance for use alone with epoxys, or in combination with the substituted urea curing agents or the boron trifluoride-amine complexes. Among these can be mentioned, for example, polyamino aromatic compounds, such as m-phenylenediamine, 4,4'-diaminodiphenyl methane and 3,3'diaminodiphenyl sulfone, as well as the aminobenzoates disclosed in U.K. Pat. No. 1,182,377, U.S. Pat. No. 3,932,360, and in Gillham, Organic, Coatings and Applied Polymer Science Proceedings, Vol. 46, p 592-598, March-April 1982. Such aromatic polyamines are effective as curing agents for a variety of polyepoxides, and the resulting cured compositions are useful as films, moldings, coatings and glass-reinforced laminates. While there is generally no indication in the properties presented in the prior art that the curing agents exemplified therein will produce the combination of toughness and strength under hot/wet conditions essential for use in the above-mentioned structural applications, the said copending application Ser. No. 518,879 describes fiber resin matrices comprising reinforcing filaments in a heat-curable epoxy resin composition comprising an epoxy prepolymer and a novel family of aromatic polyamine curing agents. The application describes neat resin formulations having, after cure, improved physical properties, e.g., higher elongation and satisfactory hot/wet modulus. Moreover, the epoxy compositions, cured with filaments, and catalyzed, exhibit improved inter-laminar toughness and residual compression strength after impact, while maintaining compression strength under hot/wet conditions.
It has now been discovered that a particular family of bisurea catalysts can be obtained by reacting an aryl monoisocyanate with an organic diamine, and that their use with epoxys alone or in further combination with polyamines provides desirable temperature-viscosity profiles during cure and unexpected resistance to degradation in hot/wet properties, as well as retention of thermal properties.